1. Field of the Invention
The present invention relates to lithium secondary batteries.
2. Description of Related Art
In recent years, lithium secondary batteries using a non-aqueous electrolyte and performing charge-discharge operations by transferring lithium ions between positive and negative electrodes have been utilized as a new type of high power, high energy density secondary battery.
As for electrodes for such lithium secondary batteries, some research has been conducted on electrodes that use a material capable of alloying with lithium as its negative electrode active material. One example of the material capable of alloying with lithium that has been studied is silicon. However, a problem with the material such as silicon capable of alloying with lithium has been that the volume of the active material expands and shrinks when it intercalates and deintercalates lithium, causing the active material to pulverize or peel off from the current collector as the charge-discharge process is repeated. As a consequence, the current collection performance in the electrode lowers, degrading the battery's charge-discharge cycle performance.
Electrodes for lithium secondary batteries that employ silicon as the active material and exhibit good charge-discharge cycle performance have been proposed in the following published PCT applications, which are assigned to the assignee of the present invention. Published PCT application No. WO 01/029913 proposes an electrode in which a non-crystalline silicon thin film is formed on a current collector by a thin-film forming technique, such as sputtering, chemical vapor deposition (CVD), or evaporation. Published PCT application No. WO 02/071512 proposes an electrode for lithium secondary batteries in which an element such as cobalt is added to silicon.
By using the electrodes proposed in the just-mentioned publications, good charge-discharge cycle performance can be attained even when a material that alloys with lithium is used as the negative electrode active material.
Nevertheless, the lithium secondary battery having a structure in which electrodes are wound around and accommodated in a battery case has a problem of poorer charge-discharge cycle performance than that employing a flat-shaped electrode.